Hard facing material for rock bits

ABSTRACT

An improved hard facing for teeth and other surfaces of milled tooth rock bits comprises steel in the range of from 18 to 32 percent by weight, and filler in the range of from 68 to 82 percent by weight. The filler is cemented tungsten carbide and cast tungsten carbide free, and includes greater than 95 percent by weight single crystal monotungsten carbide particles. The single crystal monotungsten carbide particles have a particle size in the range of from 200 to 500 mesh and, preferably in the range of from 200 to 325.

FIELD OF THE INVENTION

This invention relates to improved wear-resistant hard facingcompositions applied to wear surfaces on teeth on bits for drilling oilwells or the like.

BACKGROUND OF THE INVENTION

Bits for drilling oil wells and the like commonly have a steel bodywhich is connected at the bottom of a drill string. Steel cutter conesare mounted on the body for rotation and engagement with the bottom of ahole being drilled to crush, gouge, and scrape rock for drilling thewell. One important type of rock bit referred to as a milled tooth bithas roughly triangular teeth protruding from the surface of the cone forengaging the rock. The principal faces of such a milled tooth thatengage the rock are usually dressed with a layer of hard facing materialto resist wear. The specific tooth geometry forms no part of thisinvention.

Conventional hard facing usually comprises particles of tungsten carbidebonded to the steel teeth by a metal alloy. In effect, the carbideparticles are suspended in a matrix of metal forming a layer on thesurface. Most hard facing on rock bits employs steel as the matrix,although other alloys may also be used.

It is quite common in referring to the material in the hard facingmerely as "carbide" without characterizing it as tungsten carbide. Themetal carbide principally used in hard facing is tungsten carbide. Smallamounts of tantalum carbide and titanium carbide may be present,although considered to be deleterious. It will be understood that asused herein, reference merely to "carbide" means tungsten carbide.

A typical technique for applying hard facing to the teeth on a rock bitis by oxyacetylene or atomic hydrogen welding. A welding "rod" or stickis formed of a tube of mild steel sheet enclosing a filler which isprimarily carbide particles. The filler may also include deoxidizer forthe steel, flux and a resin binder. The hard facing is applied bymelting an end of the rod on the face of the tooth. The steel tube meltsto weld to the steel tooth and provide the matrix for the carbideparticles in the tube. The deoxidizer alloys with the mild steel of thetube.

Three types of tungsten carbide have been employed for hard facing.Possibly the most common is crushed cast carbide. Tungsten forms twocarbides, WC and W2C and there can be an essentially continuous range ofcompositions therebetween. Cast carbide is typically a eutectic mixtureof the WC and W2C compounds, and as such is substoichiometric, that is,it has less carbon than the more desirable WC form of tungsten carbide.Cast carbide is frozen from the molten state and comminuted to thedesired particle size.

Another type of tungsten carbide is so-called macrocrystalline tungstencarbide. This material is essentially stoichiometric WC in the form ofsingle crystals. Most of the macrocrystalline tungsten carbide is in theform of single crystals. When larger particle sizes are examined, it isfound that some bicrystals of WC are formed. Macrocrystalline WC isdesirable for its toughness and stability.

The third type of tungsten carbide used in hard facing comprisescemented tungsten carbide, sometimes referred to as sintered tungstencarbide. Cemented tungsten carbide comprises small particles of tungstencarbide (e.g., 1 to 15 microns) bonded together with cobalt. Cementedtungsten carbide is made by mixing tungsten carbide and cobalt powders,pressing the mixed powders to form a green compact, and "sintering" thecomposite at temperatures near the melting point of cobalt. Theresulting dense cemented carbide can then be comminuted to formparticles of cemented tungsten carbide for use in hard facing.

Although mild steel sheet is used when forming the tubes, the steel inthe hard facing as applied to a rock bit is a hard, wear resistant,alloy steel. This occurs by reason of the oxidizers such as silicon andmanganese mixed in the filler in the tube and dissolution of tungsten,carbon, and possibly cobalt, from the tungsten carbide during welding.There may also be some mixing with alloy steel from the teeth on thecone.

It is important to provide as much wear resistance as possible on theteeth of a rock bit cutter cone. The effective life of the cone isenhanced as wear resistance is increased. It is desirable to keep theteeth protruding as far as possible from the body of the cone since therate of penetration of the bit into the rock formation is enhanced bylonger teeth (however, unlimited length is infeasible since teeth maybreak if too long for a given rock formation). As wear occurs on theteeth, they get shorter and the drill bit may be replaced when the rateof penetration decreases to an unacceptable level. It is desirable tominimize wear so that the footage drilled by each bit is maximized. Thisnot only decreases direct cost, but also decreases the frequency ofhaving to "round trip" a drill string to replace a worn bit with a newone.

U.S. Pat. No. 4,944,774 discloses a hard facing material for use withthe teeth of rock bits that comprises a mixture of crushed cementedtungsten carbide having a particle size in the range of from 20-30 mesh,and macrocrystalline tungsten carbide having a particle size in therange of from 40-80 mesh. Such a hard facing material is known toprovide a good degree of wear resistance and abrasion protection of theinner teeth, and somewhat improved wear resistance of the gage surfacesof the cone and gage row of teeth.

Due to the unique wear encountered on the gage surfaces of the cone andteeth along the hole wall, it is desired that an improved hard facingmaterial be developed for use in providing improved wear resistance andabrasion protection for such gage surfaces, and for other non-gage teethsurfaces as well. Advances in wear resistance of hard facing aredesirable to increase the duration during which a hole diameter can bemaintained, to enhance the footage a drill bit can drill before becomingdull, and to enhance the rate of penetration of such drill bits. Suchimprovements translate directly into reduction of drilling expense.

BRIEF SUMMARY OF THE INVENTION

There is, therefore, provided in practice of this invention according toa presently preferred embodiment, hard facing materials for both aninner row of rock bit teeth, and gage surfaces of a rock bit cone andgage row of rock bit teeth. A hard facing material for the inner row ofrock bit teeth comprises a blend of types of tungsten carbide, and ahigher proportion of tungsten carbide relative to the binder steel thanhad previously been considered feasible. The composition for such hardfacing comprises more than 68% by weight filler and a balance of steel.Preferably the steel is present in the range of from 18% to 32% byweight, with filler in the range of from 68% to 82% by weight. Thefiller preferably comprises from 20% to 35% by weight single crystalmonotungsten carbide, WC, from 65% to 80% by weight particles ofcemented tungsten carbide, and a balance of steel deoxidizer.

An improved hard facing for the gage surfaces of the cone and gage rowteeth comprises steel in the range of from 18 to 32 percent by weight,and filler in the range of from 68 to 82 percent by weight. The filleris cemented WC and cast WC free, and includes greater than 95 percent byweight single crystal monotungsten carbide particles, and a balance ofdeoxidizer. The single crystal monotungsten carbide particles have aparticle size primarily less than about 200 mesh. The improved hardfacing material displays an improvement in wear/abrasion resistance ofup to 65 percent when compared to prior types of hard facing materialsemploying cemented tungsten carbide, cast tungsten carbide, and blendsof cemented, cast, and single crystal tungsten carbides.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated as the same becomes better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings wherein:

FIG. 1 is a perspective view of a milled tooth rock bit constructedaccording to principles of this invention; and

FIG. 2 is a fragmentary cross section of an exemplary tooth on such arock bit.

DESCRIPTION

Art exemplary milled tooth rock bit comprises a stout steel body 10having a threaded pin 11 at one end for connection to a conventionaldrill string. At the opposite end of the body there are three cuttercones 12 for drilling rock for forming an oil well or the like. Each ofthe cutter cones is rotatably mounted on a pin (hidden) extendingdiagonally inwardly on one of the three legs 13 extending downwardlyfrom the body of the rock bit. As the rock bit is rotated by the drillstring to which it is attached, the cutter cones effectively roll on thebottom of the hole being drilled. The cones are shaped and mounted sothat as they roll, teeth 14 on the cones gouge, chip, crush, abrade,and/or erode the rock at the bottom of the hole. The teeth 14G in therow around the heel of the cone are referred to as the gage row teeth.They engage the bottom of the hole being drilled near its perimeter on"gage." Fluid nozzles 15 direct drilling mud into the hole to carry awaythe particles of rock created by the drilling.

Such a rock bit is conventional and merely typical of variousarrangements that may be employed in a rock bit. For example, most rockbits are of the three cone variety illustrated. However, one, two andfour cone bits are also known. The arrangement of teeth on the cones isjust one of many possible variations. In fact, it is typical that theteeth on the three cones on a rock bit differ from each other so thatdifferent portions of the bottom of the hole are engaged by the threecutter cones so that collectively the entire bottom of the hole isdrilled. A broad variety of tooth and cone geometries are known and donot form a specific part of this invention.

Exemplary teeth on such a cone are generally triangular in across-section taken in a radial plane of the cone. Such a tooth has aleading flank 16 and trailing flank 17 meeting in an elongated crest 18.The flanks of the teeth are covered with a hard facing layer 19.Sometimes only the leading face of each tooth is covered with a hardfacing layer so that differential erosion between the wear-resistanthard facing on the front flank of a tooth and the less wear-resistantsteel on the trailing face of the tooth tends to keep the crest of thetooth relatively sharp for enhanced penetration of the rock beingdrilled.

The leading face of the tooth is the face that tends to bear against theundrilled rock as the rock bit is rotated in the hole. Because of thevarious cone angles of teeth on a cutter cone relative to the angle ofthe pin on which the cone is mounted, the leading flank on the teeth inone row on the same cone may face in the direction of rotation of thebit, whereas the leading flank on teeth in another row may on the samecone face away from the direction of rotation of the bit. In othercases, particularly near the axis of the bit, neither flank can beuniformly regarded as the leading flank and both flanks may be providedwith a hard facing.

There are also times when the ends of a tooth, that is, the portionsfacing in more or less an axial direction on the cone, are also providedwith a layer of hard facing. This is particularly true on the so-calledgage surface of the bit which is virtually always provided with a hardfacing. The gage surface is a generally conical surface at the heel of acone which engages the side wall of a hole as the bit is used. The gagesurface includes the outer end of teeth 14G in the so-called gage row ofteeth nearest the heel of the cone and may include additional areanearer the axis of the cone than the root between the teeth. The gagesurface is not considered to include the leading and trailing flanks ofthe gage row teeth. The gage surface encounters the side wall of thehole in a complex scraping motion which induces wear of the gagesurface. In some embodiments, hard facing may also be applied on theshirttail 20 at the bottom of each leg on the bit body.

Such structure of a milled tooth rock bit is well known and does notform a specific portion of this invention, which relates to the specifichard facing material employed on the teeth of a milled tooth cuttercone.

Thus, in practice of this invention, the hard facing material comprisesa mixture of relatively larger particles of cemented tungsten carbideand relatively smaller particles of single crystal monotungsten carbide,WC. The carbide particles are in a matrix of alloy steel welded to thealloy steel of the teeth of the cutter cone.

As used herein, cemented tungsten carbide refers to a material formed bymixing particles of tungsten carbide, typically monotungsten carbide,and particles of cobalt or other iron group metal, and sintering themixture. In a typical process for making cemented tungsten carbide,carbide and cobalt particles are vigorously mixed with a small amount oforganic wax which serves as a temporary binder. An organic solvent maybe used to promote uniform mixing. The mixture may be prepared forsintering by either of two techniques: it may be pressed into solidbodies often referred to as green compacts; alternatively, it may beformed into granules or pellets such as by pressing through a screen, ortumbling and then screened to obtain more or less uniform pellet size.

Such green compacts or pellets are then heated in a vacuum furnace forfirst evaporating the wax and then to a temperature near the meltingpoint of cobalt (or the like) which causes the tungsten carbideparticles to be bonded together by the metallic phase. After sintering,the compacts are crushed and screened to a desired particle size. Thecrushed cemented carbide is generally much more angular than the pelletswhich tend to be rounded. The sintered pellets tend to bond togetherduring sintering and are crushed to break them apart. These are alsoscreened to obtain a desired particle size. Cemented tungsten carbidefrom such compacts may be made specifically for use in hard facing, maybe manufacturing scrap from making other products, or may be scrap fromworn out tungsten carbide products crushed and screened for thispurpose.

Single crystal monotungsten carbide is commercially available fromKennametal, Inc., Fallon, Nev. This material is sometimes known asmacro-crystalline tungsten carbide.

This material is to be distinguished from so-called cast tungstencarbide. Cast tungsten carbide has approximately the eutecticcomposition between bitungsten carbide, W2C, and monotungsten carbide,WC. The cast carbide is typically made by resistance heating tungsten incontact with carbon in a graphite crucible having a hole through whichthe resultant eutectic mixure drips. The liquid is quenched in a bath ofoil and is subsequently comminuted to a desired particle size. Castcarbide is brittle due to residual stresses from this thermal treatmentand, when used in a hard facing composition attached by welding withalloy steel, may deplete carbon from the steel since the carbon contentis substoichiometric with respect to the stable WC.

Hard facing is applied to the teeth and gage surface by welding with a"rod" in the form of a mild steel tube containing either the particlesof mixed cemented tungsten carbide and single crystal WC, or theparticles of single crystal WC. However, it is to be understood withinthe scope of this invention that methods other than that specificallydescribed can be used to apply the hard facing material of thisinvention.

A hard facing material for hard facing the inner rows of teeth comprisesa mixture of crushed cemented or sintered tungsten carbide particleshaving one or more different range of particle size, crushed casttungsten carbide particles, macrocrystalline tungsten carbide,deoxidixer and resin binder recited in U.S. Pat. No. 4,944,744, which ishereby enclosed by reference.

A composition within the tube for hard facing inner rows of teeth, thatis, rows other than the gage row and surfaces of the cone other than thegage surface, employs 20 to 30 mesh cemented tungsten carbide. In anexemplary embodiment, there is a minimum of 65% of the carbide particlesretained on a 30 mesh screen. No more than 10% is retained on a 20 meshscreen, and no more than 25% passes through the 30 mesh screen. None ofthe particles are larger than about 14 mesh. The grain size of thetungsten carbide grains in the particles of cemented tungsten carbideare in the range of from about one to fifteen microns. The bindercontent in such a cemented tungsten carbide is preferably in the rangeof from 6% to 8% by weight and is preferably cobalt. Preferably thematerial is substantially free of tantalum carbide and titanium carbide.

The single crystal WC is preferably in the range of from 40 to 80 mesh.Thus, a majority of the crystals are smaller than 40 mesh and at least80% of the crystals are larger than 80 mesh. No more than 5% of thecrystals should pass through 100 mesh screen.

The ratio of particle size of the larger particles of cemented tungstencarbide to smaller monocrystalline carbide can be in the range of fromabout two to five. A larger ratio is less desirable since the smallerparticles can be so small that excessive solution in the alloy steelmatrix may occur. A size ratio of three is preferred.

With such particle size ranges for the cemented tungsten carbide and thesingle crystal monotungsten carbide, the cemented carbide particles areabout three times as large as the single crystal WC. The 30 meshmaterial has a particle size of about 0.52 mm, and 80 mesh material hasa particle size of about 0.17 mm.

The weight ratio of the larger particle size cemented tungsten carbideto the smaller particle size single crystal WC is in the range of from35:65 to 80:20, and preferably in the range of from 65:35 to 80:20. In aparticularly preferred embodiment, the proportion of larger sizecemented tungsten carbide is 75% by weight and the smaller particle sizesingle crystal WC is 25%. A substantial proportion of the cementedcarbide is preferred for enhanced toughness of the hard facing.

In addition to the carbide in the filler in the tube, it is desirable toinclude up to five percent by weight of deoxidizer and temporary resinbinder. A suitable deoxidizer is silicomanganese obtained fromKennemetal, Inc., Fallon, Nev. The nominal composition of thesilico-manganese is 65% to 68% manganese, 15% to 18% silicon, a maximumof 2% carbon, a maximum of 0.05% sulfur, a maximum of 0.35% phosphorus,and a balance of iron. Preferably about four percent deoxidizer is used.A small amount of thermoset resin is desirable for partially holding theparticles together in the tube so that they do not simply fall outduring welding. A half percent is adequate.

An exemplary filler composition can be made up using 25 kg of 40 to 80mesh single crystal monotungsten carbide, 75 kg of 20 to 30 mesh crushedcemented tungsten carbide, 4 kg of silico-manganese deoxidizer, and 0.5kg of phenolic resin binder. The particles are coated with the resinsuspended in an alcohol solution which is then dried. After the tubesare loaded with the filler and the ends crimped, the ends are dipped ina solution of phenolic resin in alcohol to add some binder at the ends.The binder is then heat cured to temporarily bind the particlestogether.

The proportion of filler to the weight of the steel tube within which itis enclosed in an exemplary embodiment is 70% to 80% filler and 20% to30% tube. These proportions can vary by plus or minus 2%. Thus, theweight of filler is in the range of from 68% to 82% and the weight ofthe tube is in the range of from 18% to 32%. This results in a higherproportion of carbide in the hard facing than in previous hard facingmaterials where the weight ratio of the carbide to the steel is about60: 40.

To obtain a weight ratio of filler to steel of 70:30, a 5/32 inch (4 mm)diameter tube is made with steel sheet having a thickness of 0.017 inch(0.43 mm). Roughly the same proportions are obtained in a 3/16 inch (4.5mm) diameter tube by making it with steel sheet 0.02 inch (0.5 mm)thick.

The hard facing material is applied to the faces of the tooth by heatingthe face to a welding temperature by an oxyacetylene or atomic hydrogentorch. When a suitable temperature is reached, the above-describedtubular welding "rod" is melted onto the face of the tooth. In anexemplary embodiment, the thickness of the hard facing layer is about1/16 to 3/32 inch (1.6 to 2.4 mm) Dissolution of the silico-manganese inthe mild steel of the tube, possible dissolution of some of thetungsten, carbon, and cobalt of the carbides, and mixing of metal fromthe body of the cutter cone results in an alloy steel matrix for thecarbide particles. Microscopic examination after the cutter cone iscarburized, quenched, and tempered indicates a Martensitic phasetransformation in the alloy steel matrix of the hard facing.

The hard facing provided in practice of this invention has proved to bemore wear-resistant on the inner row teeth of milled tooth cutters thanthe prior hard facing employing single crystal WC. Comparisons were madeby hard facing alternate teeth on a cutter cone with the prior hardfacing materials and with the improved hard facing material provided inpractice of this invention. In every bit where this was done, the teethhaving the improved hard facing was as good as or better than the priorhard facing. In many bits prior hard facing showed a much greater amountof wear than the teeth having improved hard facing formed of a highproportion of relatively larger particles of cemented tungsten carbideand relatively smaller particles of single crystal monotungsten carbide.

The improvement in performance of the hard facing translates directlyinto increased footage of well drilled and increased rate ofpenetration, both of which translate directly into lowered costs for thedriller.

The enhanced performance may arise from a variety of factors. Generallyspeaking, the cemented tungsten carbide is tougher than either the castcarbide or the single crystal carbide. Thus, having relatively largeparticles of cemented tungsten carbide provides a toughness to the hardfacing which resists breakage. Further, the single crystal monotungstencarbide is a tougher material than the cast carbide which is subject toresidual stresses, and even cracks, due to rapid quenching from hightemperature and subsequent comminution.

Further, the single crystal monotungsten carbide is harder than thecemented carbide and therefore more resistant to wear. It also providesa hard material with sharp edges for effective cutting of rock formationas the rock bit is used, the cemented carbide tending to be more roundedand with fewer sharp edges. By mixing relatively larger particles ofcemented tungsten carbide with relatively smaller particles of singlecrystal tungsten carbide, denser packing of the carbide particles can beobtained than when there is no difference in particle size. Thus, theproportion of carbide to steel in the hard facing material can behigher, nominally, about six to ten percent higher.

The proportion of carbide in the hard facing is determined largely bythe proportion in the welding "rod" used for applying the hard facing.Some dilution may occur by alloy steel from the surface of the tooth onthe cutter cone. This dilution is not a large contributor since in atypical application of hard facing to a milled tooth cutter cone for arock bit, the thickness of hard facing is in the order 2 mm. The amountof dilution depends to some extent on the technique employed by thewelder applying the hard facing.

The carbide content in the hard facing can be estimated bymetallographic examination of a cross section through the hard facing.The approximate areas of the carbide and binder phases can bedetermined. From this, the volume percentages of binder and carbide canbe estimated, and in turn the weight percentages. Since use ofdeoxidizer in the filler of a welding tube is essential to producingvoid free binder phase, the dilution of the carbide filler can be takeninto account and the ratio of filler weight to tube weight approximated.A hypothetical tube type welding rod can be projected from a hard facingdeposited on the surface by other techniques.

Thus, for consistency in this specification, the proportion of carbideto alloy steel in the hard facing is considered on the basis of carbidecontent in the stick used to melt the hard facing onto the surface. Aspointed out above, the filler of carbide, binder and deoxidizer is 70%to 80% by weight (plus or minus 2%) of the stick and the mild steel tubeis 20% to 30% by weight (plus or minus 2%). The filler is about 96%carbide (plus or minus 2%), with a balance of deoxidizer and binder.Thus, as deposited, the carbide content, if both "minus" tolerances wereto occur could be as low as 64% by weight, and if both "plus" tolerancelevels occurred could be as high as 79% by weight. It is generally foundin practice that the actual carbide content of the hard facing as foundon the faces of the teeth on the rock bit after welding is more than 65%by weight and preferably 72% or more. However, regardless of suchfactors, as used in this specification, the carbide content is referredto as the filler content of a tube used to weld the hard facing on theface of the rock bit tooth.

The high packing density of the relatively larger cemented tungstencarbide particles and relatively smaller single crystal carbideparticles is appropriate for resisting hypothesized wear mechanisms forhard facing material. One mechanism for wear is believed to be fractureof carbide particles. Tougher carbide such as cemented tungsten carbideand single crystal monotungsten carbide enhance resistance to this wearmechanism.

The other postulated wear mechanism comprises "extrusion" or yieldingand consequent wear of the binder phase securing the carbide particlesto the substrate. Wear of the binder leaves carbide particles exposedand unsupported for possible fracture. One way of enhancing wearresistance of the binder is to make it stronger and harder. An alloysteel binder as used in practice of this invention provides suchhardness and strength while retaining sufficient toughness to keep thehard facing intact.

Another way of enhancing wear resistance of the binder is to reduce themean distance between particles so that the binder layer is thinner.This can be done by having smaller particles, but this may diminish thecutting ability of the teeth on the cutter cone. The enhanced packingdensity and higher proportion of carbide to binder provided in practiceof this invention also reduce the mean distance between particles orthickness of the binder phase which may be subject to deformation andwear.

In portions of a rock bit where abrasion by rock formation is a moresignificant wear mechanism than impact of rock surfaces on the hardfacing, a high proportion of single crystal WC may be employed withthree times the weight percentage of larger particles than smallerparticles, and with the larger particles being three times as large asthe smaller particles. This provides a suitable particle sizedistribution for reducing the mean free path of binder between adjacentparticles. In this way the abrasion resistance of the hard singlecrystal WC can be advantageously combined with the resistance of thebinder to extrusion and wear that would leave the carbide unsupported.

Generally speaking, the proportion of carbide to steel in the hardfacing should be maximized for best wear resistance. This desideratum ispromoted by employing two different size particles for enhanced packingdensity. This tends to decrease toughness of the hard facing. Toughnessis maintained by employing larger particles of cemented carbide andsmaller particles of monocrystalline carbide which have greatertoughness than cast carbide. The properties of the hard facing in theharsh environment encountered by a rock bit are not simple functions ofparticle size and proportion since there is interaction with the matrixas well.

An improved hard facing material for hard facing the gage surfaces ofthe cone and gage row teeth comprises ultra-fine particles ofmacrocrystalline tungsten carbide, deoxidixer and resin binder. Theimproved hard facing material is free of both cemented tungsten carbideand cast tungsten carbide.

A composition within the tube for hard facing the gage surfaces of thecone and gage rows of teeth comprises single crystal WC having aparticle size primarily of less than about 200 mesh, preferably in therange of from about 200 to 500 mesh, and more preferably in the range offrom 200 to 325 mesh. Thus, in a preferred embodiment, 80% or more ofthe crystals are smaller than about 200 mesh. In a preferred embodimentcomprising 200 to 325 mesh single crystal WC particles, the singlecrystal WC has an average particle size in the range of from about 30 to70 micrometers.

It is desired that the filler comprise greater than about 95 percent byweight single crystal WC. In addition to the single crystal WC in thefiller in the tube, it is desirable to include up to five percent byweight of deoxidizer and temporary resin binder. Suitable deoxidizersare those previously described above for the hard facing materialmixture. Preferably about four percent deoxidizer is used. A smallamount of thermoset resin is desirable for partially holding theparticles together in the tube so that they do not simply fall outduring welding. A half percent is adequate. In a preferred embodiment,the filler comprises in the range of from 95 to 98 percent by weightsingle crystal WC.

An exemplary filler composition can be made up using 100 kg of 200 to325 mesh single crystal monotungsten carbide, 4 kg of silico-manganesedeoxidizer, and 0.5 kg of phenolic resin binder.

The proportion of filler to the weight of the steel tube within which itis enclosed in an exemplary embodiment is 70% to 80% filler and 20% to30% tube. These proportions can vary by plus or minus 2%. Thus, theweight of filler is in the range of from 68% to 82% and the weight ofthe tube is in the range of from 18% to 32%.

To obtain a weight ratio of filler to steel of 70:30, a 5/32 inch (4 mm)diameter tube is made with steel sheet having a thickness of 0.017 inch(0.43 mm). Roughly the same proportions are obtained in a 3/16 inch (4.5mm) diameter tube by making it with steel sheet 0.02 inch (0.5 mm)thick.

The improved hard facing material is applied to the gage surfaces of thecone and gage row teeth in the same manner previously described forapplying the hard facing mixture to the inner row teeth.

The improved hard facing provided in practice of this invention is morewear-resistant on the gage surfaces of the cone and gage row teeth ofmilled tooth cutters than hard facings employing cemented or casttungsten carbide particles, mixtures of such particles with singlecrystal WC particles, or single crystal WC particles having particlesizes above about 200 mesh. ASTM G65 low speed abrasion tests show thatthe improved hard facing materials of this invention display up to 65percent improvement in wear/abrasion resistance, measured as materialvolume, when compared to embodiments of the hard facing materialsdescribed above used for the inner row teeth.

The improvement in performance of the improved hard facing is believeddue to the contribution of the ultra-fine particle size of the singlecrystal WC particles to form a facing surface having an ultra-fine grainstructure that is highly wear/abrasion resistant. Generally speaking,the use of such ultra-fine particles in forming the improved hard facingmaterial produces a hard facing having extremely good wear and abrasionresistance, but having reduced toughness. Hard facing materials formedfrom larger single crystal WC particles have better toughness anddisplay less breakage.

For this reason it is desired that the improved hard facing material beused on the gage surfaces of the cone and gage row teeth, to provideimproved resistance against the aggressive wear and abrasion caused bycontinuous rubbing against the hole wall. However, it is to beunderstood that the improved facing material can be used on non-gagesurfaces of the cone and cone teeth, including inner row teeth,depending on such factors as the nature of the subterranean material,drilling conditions, and the nature of hole being drilled.

Other modifications and variations of hard facing for a rock bit will beapparent to one skilled in the art. It is, therefore, to be understoodthat within the scope of the appended claims, this invention may bepracticed otherwise than as specifically described.

What is claimed is:
 1. A rock bit comprising:a body; at least onecutting cone rotatably mounted to an end of the body, wherein the coneincludes a gage surface at a heel portion of the cone; a number of teethon the cone, wherein the teeth include gage row teeth located near aheel of each cone, wherein the gage surface of the cone and an outer endof the gage row teeth include a rock bit hard facing comprising: steelin the range of from 18 to 32 percent by weight; filler in the range offrom 68 to 82 percent by weight, the filler comprising:at least 95percent by weight single crystal monotungsten carbide particles; and abalance of deoxidizer; wherein the teeth include inner row teeth locatednear a center of the cone, and wherein the inner row teeth include ahard facing material comprising: steel in the range of from 18 to 32percent by weight; and filler in the range of from 68 to 82 percent byweight, the filler comprising:from 20 to 35 percent by weight singlecrystal monotungsten carbide particles; from 65 to 80 percent by weightcemented tungsten carbide particles; and a balance deoxidizer.
 2. A rockbit as recited in claim 1 wherein the single crystal monotungstencarbide particles of the hard facing material on the inner row teethhave a particle size primarily between 40 and 80 mesh.
 3. A rock bit asrecited in claim 1 wherein the single crystal monotungsten carbideparticles and the cemented tungsten carbide particles of the hard facingmaterial on the inner row teeth have an average particle size in therange of from 200 to 500 micrometers.
 4. A rock bit as recited in claim1 wherein the cemented tungsten carbide particles of the hard facingmaterial on the inner row teeth have a particle size primarily between20 and 30 mesh.
 5. A rock bit as recited in claim 1 wherein the fillerfor forming the gage surface of the cone and an outer end of the gagerow teeth comprises in the range of from 95 to 98 percent by weightsingle crystal monotungsten carbide particles.
 6. A rock bit as recitedin claim 1 wherein the single crystal monotungsten carbide particles ofthe hard facing material on the gage surface of the cone and an outerend of the gage row teeth have an average particle size in the range offrom 30 to 70 micrometers.
 7. A rock bit as recited in claim 1 whereinthe single crystal monotungsten carbide particles of the hard facingmaterial on the gage surface of the cone and an outer end of the gagerow teeth have a particle size in the range of from 200 to 325 mesh. 8.A rock bit comprising:a body; at least one cutting cone rotatablymounted to an end of the body, wherein the cone includes a gage surfaceat a heel portion of the cone; a number of teeth on the cone, whereinthe teeth include gage row teeth located near a heel of each cone and aninner row of teeth located near a center of the cone, wherein the gagesurface of the cone and at least an outer end of the gage row teethinclude a hard facing comprising a mixture of steel and filler, whereinthe filler is free of both cemented tungsten carbide and crushedtungsten carbide and comprises single crystal monotungsten carbideparticles and a balance of deoxidizer; wherein the inner row of teethinclude a hard facing material comprising a mixture of steel and filler,wherein the filler comprises:steel in the range of from 18 to 32 percentby weight; and filler in the range of from 68 to 82 percent by weight,the filler comprising:from 20 to 35 percent by weight single crystalmonotungsten carbide particles having a particle size primarily between40 and 80 mesh; from 65 to 80 percent by weight cemented tungstencarbide particles; and a balance deoxidizer.
 9. A rock bit as recited inclaim 8 wherein the cemented tungsen carbide particles have a particlesize primarily between 20 and 30 mesh.
 10. A rock bit as recited inclaim 8 wherein the hard facing material for the gage surface of thecone and at least outer end of the gage row teeth comprises in the rangeof from 18 to 32 percent by weight of the steel, and in the range offrom 68 to 82 percent by weight of the filler, the filler comprising inthe range of from 95 to 98 percent by weight of the single crystalmonotungsten carbide particles having a particle size primarily lessthan 200 mesh.
 11. A rock bit comprising:a body; a cutting conerotatably mounted to an end of the body, wherein the cone includes agage surface at a heel portion of the cone; a number of teeth on thecone, wherein the teeth include gage row teeth located near a heel ofeach cone, wherein the gage surface of the cone and an outer end of thegage row teeth include a rock bit hard facing comprising a mixture ofsteel and filler, wherein the filler is free of both cemented tungstencarbide and east tungsten carbide and comprises single crystalmonotungsten carbide particles and deoxidizer; wherein the teeth includeinner row teeth located near a center of the cone and include a hardfacing material comprising a mixture of steel and filler, wherein thefiller comprises single crystal monotungsten carbide particles, cementedtungsten carbide particles, and a balance deoxidizer.
 12. A rock bitcomprising:a body; at least one cutting cone rotatably mounted to an endof the body, wherein the cone includes a gage surface at a heel portionof the cone; a number of teeth on the cone, wherein the teeth includegage row teeth located near a heel of each cone and an inner row ofteeth located near a center of the cone, wherein the gage surface of thecone and at least an outer end of the gage row teeth include a hardfacing comprising a mixture of steel and filler, wherein the filler isfree of both cemented tungsten carbide and crushed tungsten carbide andcomprises in the range of from 95 to 98 percent by weight single crystalmonotungsten carbide particles and a balance deoxidizer; wherein theinner row of teeth include a hard facing material comprising a mixtureof steel and filler, wherein the filler comprises a mixture of singlecrystal monotungsten carbide particles, cemented tungsten carbideparticles, and a balance deoxidizer.
 13. A rock bit as recited in claim12 wherein the single crystal monotungsten carbide particles used formaking the hard facing filler for the gage surface of the cone and theat least outer end of the gage row teeth have a particle size in therange of from 300 to 500 mesh.
 14. A rock bit as recited in claim 12wherein the hard facing material for the hard facing filler for the gagesurface of the cone and the at least outer end of the gage row teethcomprise in the range of from 18 to 32 percent by weight of the steel,and in the range of from 68 to 82 percent by weight of the filler.